The present invention relates to fabrication of MOS transistors with reduced susceptibility to channel hot carrier effects, more specifically, to deuterium passivation of a gate interface.
Background: Deuterium Passivation
Hydrogen passivation of the dangling bonds which form electrically active interface traps improves device function, but hydrogen passivation can be degraded by subsequent hot electron impact. Recent results have shown that passivation with deuterium instead of hydrogen in the post-metal anneal process is more stable and therefore less likely to succumb to hot electron degradation, resulting in an improvement in channel hot carrier lifetime of 10-100.times.. I. C. Kizilyalli et al., "Deuterium Post-Metal Annealing of MOSFET's for Improved Hot Carrier Reliability," 18 IEEE Electron Device Letters, 81 (1997); J. W. Lyding et al., "Reduction of Hot Electron Degradation in Metal Oxide Semiconductor Transistors by Deuterium Processing," 68 Applied Physics Letters 18 (1996); 68 Applied Physics Letters 2526 (1996); C. G. Van de Walle and W. B. Jackson, "Comment on `Reduction of hot electron degradation in metal oxide semiconductor transistors by deuterium processing`"; Applied Physics Letters 69 (1996) 2441, all of which are hereby incorporated by reference.
Published work on deuterium sintering to date has been done on devices that have been pulled after the metal-1 metallization step. However, typical process flows for conventional CMOS devices require many additional levels of metallization and isolation dielectric steps which are performed at high temperatures. On the average, Si--H or Si--D bonds are broken at temperatures between 500 degrees Celsius and 550 degrees Celsius although a smaller portion of these bonds may be broken at lower temperatures (e.g., down to about 300 degrees Celsius). Once free, deuterium and hydrogen are mobile and are expected to diffuse from interfacial trap sites. Thus, the subsequent thermal processing is likely to destroy any beneficial effects from the early sintering process.
Semiconductor Process with Deuterium Predominance at High Temperatures
The disclosed approach for avoiding harmful deuterium diffusion during steps executed at temperatures likely to break Si--H/Si--D bonds is to perform non-oxidizing steps which require temperatures greater than 300 degrees Celsius (or other threshold temperature) predominantly in a deuterium-enriched atmosphere (e.g., N2:D2=90:10). By providing a deuterium-enriched ambient at the lowest temperatures where hydrogen depassivation is significant, potential replacement of hydrogen with deuterium is maximized.
The advantages of providing a deuterium-enriched environment during these high temperature steps include:
compensating for depletion of deuterium from diffusion away from the gate oxide interface traps; PA1 minimizing any adverse affects on passivation due to thermal processing steps; and PA1 improving hot-channel-carrier reliability.